1. Field of the Invention
The field of the invention is related to direction indicators. More particularly, to a simple direction indicator for use in an automobile or similar vehicle for determining the direction of a source emitting sound waves above a predetermined frequency. Even more particularly, the present invention relates to the direction of an emergency vehicle having a siren emitting sound waves having a frequency above 1,000 hertz.
2. Prior Art
All too often one reads in the newspaper of an accident between an emergency vehicle, such as an ambulance, fire truck, or a police car and an automobile. The excuse usually given by the driver of the automobile is, "I didn't hear the siren", which all too often is a true statement. Modern automobiles are well sound proofed to block out street and road noises, and with the windows rolled up, the sound proofing is quite effective. Added to this fact, many of these automobiles are equipped with air conditioners which require that the windows be rolled up in both summer and winter. Most of the air conditioned automobiles lack the conventional side window vents, which normally prevent undersirable drafts and wind noises when the side windows are open. Therefore, there is a further tendency not to keep the side windows opened even in moderate weather, excluding outside sounds the year around. Finally, almost every vehicle is equipped with a radio, which is often played much too loud. Taking these factors in collectively, the operator of a vehicle is often quite oblivious to sounds originating outside the vehicle and, unfortunately, even the sirens of emergency vehicles. Even if he does hear the siren, he usually has no idea of where the emergency vehicle is, relative to his present location and his direction of travel.
The present invention is a frequency sensitive sound indicator, and in its preferred embodiment is a direction indicator sensitive to the frequency of an emergency vehicle's siren giving the operator a visual indication of the existance and direction of the source relative to his vehicle.
Audio direction indicators or finders are known in the prior art, and have been used by the military to indicate the direction of a distant gun implacement or the direction of an unseen sniper. These direction finders use two or more strategically placed microphones for detecting the sound. By knowing the exact position of the microphones, the direction of origin is determined by comparing the arrival times or phase differences of the sound incident at the different microphones. These systems are quite complex, relatively expensive and their accuracy is well above the needs for the automotive application discussed above. Closer to the disclosed invention is the "Relative Movement Responsive Siren Alert" taught by J. W. Perrin in U.S. Pat. No. 3,867,719 (February 1975), in which the doppler frequency of a siren is used to excite an indicator light visible giving the operator an indication of whether the source is approaching or retreating from the vehicle. The system, also, includes an audio alarm when the intensity of an approaching siren exceeds a predetermined value. Although the system taught by Perrin notifies the driver of an approaching emergency vehicle, the driver has no knowledge of the direction from which it is approaching. The disclosed invention overcomes this deficiency by giving the operator a further indication of the direction of the emergency vehicle.